Conventionally, an induction motor is widely used as a driving system for a railroad vehicle. However, due to energy-saving considerations, a driving system with a permanent magnet synchronous motor has recently become widespread. In a permanent magnet synchronous motor, a magnet implanted in a rotor generates a magnetic field and creates current in the rotor. Unlike an induction motor, the permanent magnet synchronous motor has no current loss in the rotor and no heat generation caused by the current loss. Accordingly, the driving system with a permanent magnet synchronous motor has higher efficiency than a driving system with an induction motor.
On the other hand, implanting the magnet in the rotor of a permanent magnet synchronous motor has negative effects. One of the negative effects is an induced voltage. The induced voltage is generated by the implanted magnet and increases commensurately to a rotating speed. During high-speed rotation, the induced voltage may exceed a DC voltage of an inverter circuit that controls the device. When the induced voltage exceeds the DC voltage of the inverter circuit, an regeneration operation is automatically started. Therefore, even when a railroad vehicle is running on inertia, it is necessary that the current be intentionally passed to suppress the induced voltage in order to weaken a magnetic flux.
Other problems occur when a motor and an inverter circuit for the motor are placed in different vehicles. In such cases, an interconnection between the vehicle of the motor and the vehicle of the inventor is necessary. Short circuits may occur in the interconnection. When a short-circuit accident or a ground fault accident occurs between the inverter circuit and the permanent magnet synchronous motor, regeneration is prevented and the magnetic flux caused by the induced voltage interferes with the normal operation of the permanent magnet synchronous motor. These problems do not occur in an induction motor.